Drafting lead and method of preparation



United States Patent ABSTRAQT OF THE DISCLOSURE This invention is directed to calcined drawing leads formed from graphite and clay and impregnated with abietyl alcohol, ester gum or combinations thereof; and to a method for producing same by subjecting the lead blanks to a vacuum at an elevated temperature followed by a pressure impregnation at an elevated temperature with a pre-heated impregnant and removal of the excess impregnant, after which the impregnated leads are cooled preparatory to further processing.

This invention relates to new and useful improvements in drafting lead adapted to be used on all drafing surfaces and more particularly seeks to provide a drafting lead that will write on plastic films in addition to the more conventional writing surfaces.

The introduction of new drawing surfaces such as matte-surfaced polyester films in addition to the conventional materials, paper, linen, tracing cloth and the like, has placed a burden on pencil manufacturers to provide universal drafting leads which may be successfully used on all such surfaces, thus avoiding the requirement of having pencils and leads of different types for different surfaces. Numerous efforts have been made to provide such a universal pencil or lead but as far as we are aware, no pencil or lead has yet proven entirely satisfactory.

Ordinary good .drafting pencils have had leads consisting of graphite, clay and wax. These have produced superior results on the older conventional drafting surfaces. They have not, however, been satifactory when used on some of the newer sheet materials, particularly surface treated polyester films, and various efforts have been made to produce a universal type of pencil. Among the modifications tried were thermoplastic leads and leads containing plastic cellulose binders.

Some of the disadvantages encountered in prior drafting leads intended, for example, for use on matte-surfaced polyester films as well as on older conventional surfaces are (1) weak points, (2) points subject to rapid wear and to mushrooming, (3) lack of opacity in the lines produced and (4) lack of uniformity and precise definition of the line boundaries.

Accordingly, it is one object of our invention to provide a drafting lead capable of satisfactory use on all drawing surfaces with respect to the opacity and definiteness of the lines produced.

It is another object of our invention to produce such a lead of high strength, free of a tendency to mushroom, and resistant to rapid wear.

It is a further object of our invention to produce drafting leads resistant to wet and dry smudging and stable under varying atmospheric conditions of temperature and humidity.

It is an additional object of our invention to produce a drafting lead having superior erasing properties and which will not scratch when used on coated or mattesurfaced polyester film.

These objects are attained by the leads of our invention having the compositions and made by the method disclosed below.

3,386,839 Patented June 4, 1968 Other objects and advantages will be apparent to those skilled in the art from the following disclosure.

According to our invention we first form leads of calcined graphite and clay and then impregnate them at an elevated temperature with a non-waxy material, which is a viscous tacky liquid or semisolid at room temperature, but which is not viscous at the impregnating temperature. The impregnating materials may, for example, be viscous tacky liquids like polyisobutylene or balsam-like resinous materials, such as abietyl alcohol, or solid resins such as ester gum, or the like, or mixtures thereof.

We have found that such viscous, tacky, non-waxy materials act as binders in the lead, reducing the amount of wear and reducing the build-up of loose particles when in use, thereby reducing smudging.

We are aware that the impregnation of drafting leads has been practiced in the past as noted above, but the impregnants have in general been of low melting point waxes or oils not of a resinous nature and insufficiently viscous and tacky to con-fer the desired properties on the lead or have lacked effective binding properties.

We have found it advantageous in making the leads of this invention to subject the first formed leads to a vacuum before impregnation to exhaust air from the insterstices of the leads, then to impregnate the leads while still under vacuum, and finally to subject the leads to superatmospheric pressure while in contact with the impregnant. In this way full-cell impregnation is achieved. Furthermore, according to our invention the impregnation is carried out at elevated temperatures above about 300 F., preferably in the neighborhood of 350 F.

Among the non-waxy impregnants which are viscous and t-acky liquids or semi-solids or resinous solids at room temperature but nonwiscous at high temperatures and suitable for use in our novel products may be mentioned, polyisobutylene, .abietyl alcohol, ester gum, glycol esters of abietic acid or combinations thereof.

The temperatures for impregnation should be in the range of 300400 F.

The following examples are intended to be illustrative only of preferred embodiments and not to limit our invention, the scope of which is defined in the appended claims.

Example 1 An assembly of leads (blanks) is first formed by conventional methods containing about 43% calcined graphite and about 57% suitable clay. These are arranged in a heated gas-tight vessel and subjected to vacuum at 250 F. After the vacuum has been maintained for one hour, the impregnant comprising abietyl alcohol which has been pro-heated to 350 F. is admitted to contact with the blanks inan amount sufficient to insure filling the pores of the lead, the temperature being raised to 350 F. in the impregnation vessel. The impregnant will normally comprise from 7% to 22% and preferably 10% to 20% by weight of the impregnated leads. The vacuum is released and air pressure applied. When impregnation is complete, the pressure is released. Then the leads are centrifuged to remove excess impregnant and then cooled for fabrication into pencils or further processed for use with mechanical pencil holders. During the vacuum phase, which lasts about 60 minute-s, the vacuum is kept at 28 to 30 inches of water and during the pressure phase, which could last up to 4 hours, the pressure is kept at to p.s.i.

Example 2 The lead blanks are of the same composition as in Example 1. The process is the same. In this case, how

ever, the impregnant comprises 90% 'ab'ietyl alcohol and 10% ester gum. The resulting lead is of a harder grade than that produced in Example 1.

Example 3 Examples 4-10 Following the process of Example 1, the following clay-graphite compositions were made up through a softhard range:

Clay Graphite 26 74 Softesl; 31. 68. 5

63 37 Hardest We claim:

1. A calcined drawing lead comprising by weight, prior to impregnation, from 37% to 74% graphite and to 22%, based on total Weight, of a material selected from the group consisting of a'bietyl alcohol, ester gum or combinations thereof.

2. A method of producing drawing leads comprising; forming calcined blanks of 37% to 74% graphite and 63% to 26% clay by Weight; subjecting the blanks to a vacuum in a closed chamber at an elevated temperature below 300 F; preheating an impregnant consisting of abietyl alcohol, an ester gum or a combination thereof to a temperature of from 300 to 400 F.; introducing the preheated impregnant into the vacuum chamber and raising the temperature therein to from 300 to 400 F.; applying superat-mospheric pressure to the impregnated leads to complete the impregnation thereof with from 7% to 22% of the impregnant based on total weight; then releasing the pressure, removing the excess impregnant and cooling the impregnated leads preparatory to further processing.

References Cited UNITED STATES PATENTS 1,723,747 7/ 19 29 Pearce ll71/l3 1,738,794 12/1929 Parker 1l71l3 2,566,753 9/l95 l Van Dusen et al 106-19 2,645,582 7/1953 Loy 106l9 2,986,472 5/1961 Murray et al, 106-19 3,010,847 11/1961 Ewing et 'al. ll7ll3 63% to 26% clay and being impregnated with from 7% 3O ALLAN LIEBERMAN, Primary Examiner. 

